A Flight Management System, or FMS, is an onboard device on an aircraft. It provides for working out a flight plan on board and its modification in the event, notably, of rerouting. A flight plan is the detailed description of the route to be followed by an aircraft within the framework of a planned flight. It includes notably a chronological sequence of waypoints described by their position, their altitude and their time of overflight. A set of waypoints and the segments connecting them form the reference trajectory to be followed by the aircraft with a view to best adhering to its flight plan. This trajectory is a valuable aid both to the ground control personnel and to the pilot, for anticipating the movements of the aircraft, for example an airplane, and thus ensuring an optimum safety level, notably in the context of the maintaining of inter-aircraft separation criteria.
There are different flight management systems depending on the type of craft and the intended application. However, certain features are common to a majority of flight management systems. Specifically, a flight management system generally comprises a navigation database, a means for recording its flight plan, a performance database for calculating a trajectory and its predictions, guidance means and interfaces on various navigation screens.
FIG. 1 represents a known flight management system 1 comprising:                databases 100 to construct trajectories and procedures, notably the demands required by the “Required Navigation Performance”, or RNP, procedure from data included in the databases such as waypoints, markers, trajectory portions called “legs”, defined by several navigation parameters or characteristics such as instructions to follow concerning a position, an altitude or a heading, for example “Navigation database NAV DB” (registered trademark),        means 101 for entering geographic elements forming the reference trajectory of the route to be followed, for example “Flight plan FPLN” (registered trademark),        databases 102 containing aerodynamic performance data and aircraft engine data, for example PERF DB, in order to calculate the predictions of altitude, time of passage, fuel consumption along the trajectory,        means 103 for constructing a continuous trajectory from points on the flight plan which meet the performance characteristics of the aircraft and the confinement constraints (RNP), for example “Lateral trajectory TRAJ” (registered trademark),        means 104 for constructing an optimized vertical profile on the lateral trajectory, for example “Prediction PRED” (registered trademark),        means 105 for calculating the geographic position of the aircraft as a function of geolocation means of the GPS (registered trademark) type, Galileo (registered trademark) type, VHF radio beacons type or inertial unit type, for example “Navigation LOCNAV” (registered trademark),        means 106 for guiding the aircraft in the lateral plane and vertical plane on its 3D trajectory, while optimizing speed, for example “GUID” (registered trademark),        means 108 for communicating with control centers 112 and other aircraft, “DATALINK”, the control centers 112 including, for example, an air traffic control center (ATC), an airline operation center (AOC), and the like.        a user interface 109 comprising a keypad and at least one screen for entering the required data and displaying the results.        
The pilot of the aircraft uses the databases 100 in collaboration with the means 101 for constructing their flight plan and connecting these various waypoints in order to work out the structure of the route to be followed by the aircraft.
The means 103 inserts the structure of the route to be followed, as worked out by the pilot, and combines this with information relating to the aircraft performance characteristics supplied by the databases 100 and 102, thus providing for defining a trajectory which meets the characteristics of the aircraft and the demands required by the RNP procedure. From this trajectory, the means 104 constructs an optimized vertical profile. The means 105 locates the aircraft wherever its position on the terrestrial globe. During the flight, the geolocation data but also the accuracy demands required by international procedures, notably RNP, are transmitted to a means 106 for assisting the pilot or the automatic flight control, enabling the aircraft to be guided on its 4D trajectory. An interface 109 enables the pilot to display this information.
The international procedure called “Required Navigation Performance”, or RNP, was first envisaged by the International Civil Aviation Organization, or ICAO, as a means for facilitating changes in airspace. The RNP was created in order that it be possible to specify the conditions to be met as regards airspace and operation, without enduring the constraints of a slow process of equipment and systems specification.
A state in collaboration with industry undertook to update the criteria for using the RNP procedures in order to solve the serious problem of access to airports located in obstacle-rich environments, or during very unfavorable meteorological conditions.
This procedure gives, in addition to the conventional indications, criteria concerning operational aspects to be taken into account for implementing operations in the air: during an engine failure, during an ascent or during a balked landing, for example.
The RNP procedure can bring about considerable advantages from the point of view of operation and safety over other procedures by prescribing an accuracy, an enhanced navigation feature to allow operations using reduced margins for overcoming obstacles which makes the implementation of approach and departure procedures possible in situations where the application of other procedures is not realizable or acceptable from an operational point of view. The RNP procedure provides for taking advantage of lateral and vertical navigation means which improve operational safety and reduce risks of impact without loss of control.
The RNP procedure authorizes, among others, an aircraft to follow a specific trajectory between two points, the trajectory being defined in three dimensions. The RNP procedure prescribes accuracy requirements that the aircraft must adhere to. For example, an RNP requirement 5 indicates that the geolocation means 105 of the aircraft must be capable of calculating the position of the aircraft in a 10 NM wide corridor, where NM means “nautical mile” and 1 nautical mile is equivalent to 1852 m.
During a flight, the geolocation accuracy demand level varies: oceanic airspaces can have an RNP demand of between 4 and 10 NM, at the start of an approach toward an airport, and RNP demands are generally between 1 and 0.5 NM and between 0.3 and 0.1 NM for precision approaches. The increase in the accuracy demand level provides for defining a trajectory in three dimensions composed of straight lines and curves in an environment with high traffic density, around areas sensitive to noise or through a difficult terrain.
The accuracy demand level required can be defined in a configuration file of the flight management system, manually by the pilot or according to the database 100 present in the flight management system. The accuracy level can also be defined by default according to whether the space flown over by the aircraft is of the oceanic type or an airport, for example.
In order to be able to follow the demands of the RNP procedure, the navigation means 105 must be capable of calculating the position of the aircraft with the required accuracy level. The guidance means 106 must also ensure a guidance capability with the same accuracy.
The accuracy level of the guidance is fixed and known for a given aircraft; however, the accuracy level calculated on the position of the aircraft varies during a flight according to whether the aircraft flies over an airport or an oceanic environment.
This is because external satellite navigation devices 111 of the Global Positioning System (GPS) type have different coverage levels depending on the geographic area in question. The same applies for radio navigation means. As regards inertial means, they suffer from the problem of inertial drift inherent to these systems.
It is down to the pilot of the aircraft to ensure that their aircraft is capable of adapting according to the demands required by the RNP procedure.
Presently, the accuracy level required with respect to the current accuracy level is not easily accessible by the pilot. Furthermore, it is known by the pilot only relative to the current position of the airplane, without a true link with the trajectory.